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Agate


Click HERE to shop agates, or scroll to the bottom of the article.

Agate is the ayurvedic birthstone for the month of May and the mystical birthstone for the month of September. 


Agate is one type of microcrystalline quartz (chalcedony). Other types of chalcedony include carnelian, chert & flint, chrysoprase, jasper, and pietersite.

Click HERE to find out about agate's energy, metaphysical properties, meanings and uses.

Agate Properties

Mineral formula: SiO2

Mineral group: a variety of chalcedony

Crystal system: hexagonal

Crystal habit: The SiO2 in agate is made of fibrous, length-fast chalcedony, not observable by the naked eye. Banding in agate is based on changes in translucency and color. The most common kind of banding is wall-lining banding; it occurs parallel to the wall, at right angles to the crystal growth direction.

Cleavage: none

Fracture: conchoidal to subconchoidal

Color: from colorless, white, grey, pink, red, blue, yellow, purple – almost any color

Luster: waxy, dull; when polished, vitreous

Diaphaneity: transparent

Moh’s scale hardness: 6 ½ to 7

Streak: white

Specific gravity: 2.6

Named after: its type occurrence at the Achates River in southwestern Sicily.  Originally reported from Dirillo river (Achates river), Acate, Ragusa Province, Sicily, Italy.

Type locality: Achates River, Sicily.

Geologic occurrence: Very common. Lining cavities in acidic and intermediate volcanic rocks. Nodules in limestones and other sedimentary rocks.

Agates are often named based on their source locality because the particular combination of colors, patterns and textures is unique to that site. Many dealers in stones introduce fanciful names for certain agates as a marketing strategy; these names have no foundation in the mineralogic literature. Roger Pabian's "Agate Lexicon" at UNL is a good reference source of information on types of agate, click HERE to see it.

In the rock and gem trade, many stones are called “agate” that are not, technically, agate, such as “feather agate” and “fire agate.”

Can you tell the difference just by looking if a piece of microcrystalline quartz is agate? Not necessarily.

Mineralogy and Geological Occurrence

Agate is a translucent form of chalcedony, a type of microcrystalline quartz. Most agate forms in cavities in ancient acidic to intermediate extrusive igneous rocks (lavas). Characteristic forms include agate with concentric color bands, and agate with moss-like or vein-like inclusions (moss agate and dendritic agate).

When agate has a specific name, the name typically refers to its place or origin or a distinctive visual characteristic. Some names are fanciful marketing creations by rock dealers.

Some types of agate include:

Bird’s Eye (or Eye) Agate – concentric rings look like eyes

Blue Lace Agate – pale blue finely banded agate often from South Africa

Botswana Agate – from Africa, dark and light banding

Cloud Agate – greyish agate with patches of blurry inclusions

Crazy Lace Agate – agate with multi-colored, twisting and turning bands, often from Mexico.

Dendritic Agate – dark crystallites of manganese or iron form branching plant-like structures within the agate matrix. They are not fossilized plants.

Enhydro Agate – an agate nodule partially filled with water

Fire Agate – not a true agate, this is a type of chalcedony with inclusions of limonite or goethite that produce an iridescent “fire” effect.

Fortification agate – agate with sharp-angled bands that look like an aerial view of an ancient fortress.

Laguna Agate – colorful agate originally described from Ojo Laguna, Mexico.

Lake Superior Agate – perhaps the world’s oldest agate (> 1 billion years) these were spread from the Lake Superior region by glaciation.

Moss Agate – agate with greenish colored inclusions looking like moss. Possibly due to altered hornblende.

Onyx – technically black and white banded agate. The name is incorrectly used by the trade for fine-grained, banded, homogeneous rock, usually calcite, that is easy to carve (“Mexican” onyx) that can be any color.

Orpheus Agate – a relatively new and highly collectible agate find from Bulgaria. Concentric bands, intensely colored with green, golden-tan, brown, white and clear.

Sardonyx – a variety of agate with reddish-brown-orange bands accompanying black and white bands

Youngite – a variety of agate (or jasper if opaque) where the original agate is coated by drusy quartz crystals. Originally described from Guerrnsey, Platte Co., Wyoming. 

Agate typically forms in silica-rich rocks, especially volcanic rocks. Silica-rich water percolating through the porous rocks precipitate a layer of microcrystalline quartz on the walls of cavities in the rocks. Over time, continued deposition from aqueous fluids starts to fill the cavity. As the environment changes, the chemistry of the water changes, so the compositions of each layer may be somewhat different resulting in different colors. Agate can form in other rocks types, including limestones and marl, as concretions.

Agate is found around the world; click HERE for a map with identified locations of agate (from mindat.org). Oregon, Washington, Idaho, and Montana are popularly abundant sources in the US. 


                            


New Mexico Agate

With its abundance of silicic volcanic rocks, New Mexico hosts lots and lots of agate!  For a visual summary of New Mexico agate locations, click HERE.

Some of the most famous agate in New Mexico is found as geodes, notably Baker Thunger Eggs (aka Geodes) from Luna County, NM.

History of Agate Use

Due to their hardness, translucence, beautify and homogeneity, agates have been valued as a lapidary material since prehistoric times. Agates are often slabbed then cut and polished into smaller cabochons. Larger pieces are slabbed and polished into gorgeous display pieces. The hardness made agate desirable for more functional objects such as bowls or mortars and pestles.

Agate has long been beloved by royalty, as well. In particular, Queen Victoria made Scottish agate a staple in her wardrobe. This fashion choice led to an explosion in the popularity of agate jewelry during her reign.

Agate Trivia

According to the Guinness Book of World Records, the largest agate is in China and weighs in at about 135,000 lbs, and is about 800 cubic feet in volume.

A valued piece of art from antiquity, The “Rubens Vase” (shown above) was carved from a single piece of agate, most likely created for a Byzantine-age Roman emperor. 



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